When a microcomputer is used to control the air/fuel mixture in an internal combustion engine or used to control other parameters affected by the rate of air/fuel mixture burned, it is necessary to measure the mass air flow through the engine's intake manifold. The measurement can be made indirectly by measuring ambient air pressure, intake manifold air pressure, intake manifold air temperature, and throttle angle, but a direct measurement can be made more precisely and is therefore more desirable. Many current engine control systems make the direct measurement of mass air flow by measuring the electrical resistance of a heated wire placed in the intake manifold. The wire must have a high thermal coefficient of resistance where a change in the resistance of the wire is directly proportional to a change in its temperature. Heat is removed from the wire in direct proportion to the mass of the air flowing past the wire. In one method, heat, in the form of electrical power, is supplied to the wire to maintain the wire at a constant temperature/resistance. Typically, an analog-to-digital converter is used to measure the resistance of the wire, and a digital output is used to calculate the mass air flow.
Examples of these prior art mass air flow meters are discussed below with reference to FIGS. 1-3.
A General Motors Corporation publication entitled, "Mass Airflow Sensor: Ambient Temperature Compensation Design Considerations", by G. Gurtcheff et al., teaches an air flow measurement circuit as shown in FIG. 1. In the circuit of FIG. 1, resistor R.sub.s is the sensing element, resistor R.sub.t is a resistor element which compensates for changes in ambient temperature, and resistors R.sub.a, R.sub.b and R.sub.c are resistors with very low temperature coefficients of resistance. Resistors R.sub.s, R.sub.t, R.sub.a, R.sub.b and R.sub.c are arranged to form a bridge circuit, and a feedback means, comprising operational amplifier 10 and transistor 20, is designed to keep the resistance of resistor R.sub.s at a fixed resistance/temperature to balance the bridge. The feedback means is purely analog and produces a voltage V.sub.b at the emitter of transistor 20, which is applied to the bridge at point A and balances the bridge. Voltage V.sub.b is also coupled to a voltage controlled oscillator (not shown) to produce a frequency proportional to voltage V.sub.b. This frequency is then used as a measurement of mass air flow.
A Japanese publication entitled, Electronic Controlled Gasoline Injector, by H. Fujisawa et al, publisher: Kouichirou Ojima, Tokyo, 1987, teaches an air flow measuring circuit, shown in FIG. 2, which is similar to the General Motors sensor described above, wherein the output of operational amplifier 30 is an analog voltage which is applied to the bridge circuit to keep the temperature and resistance of sense element 40 constant. The voltage measured at point A on the bridge circuit, which is proportional to the analog output of operational amplifier 30, is then used to measure the air flow over sense element 40.
A paper entitled "Bosch Mass Air Flow Meter: Status and Further Aspects", by Sumal and Sauer, contained in the compendium of papers entitled, "Sensors and Actuators SP-567" (SAE, 1984), teaches a circuit, shown in FIG. 3, similar to the above-mentioned circuits, for measuring mass air flow. In FIG. 3, sense element R.sub.H and element R.sub.K are incorporated in a bridge circuit, where element R.sub.K is a temperature compensation sensor. As in the above-mentioned prior art, operational amplifier 50, having inputs coupled to bridge terminals A and B, provides an output voltage, coupled to bridge terminals C and D, which keep the resistance and temperature of sense element R.sub.H constant. The voltage U.sub.M measured across fixed resistor R.sub.3 is thus proportional to the output voltage of operational amplifier 50. Only analog voltage is generated in the circuit as in the above-mentioned prior art.
Engine design engineers are now asking for analog to digital (A/D) converters with ten or twelve bits of precision. A/D converters of this precision are expensive, especially if they must be designed to operate in the harsh environment of an automobile engine compartment. As seen, prior art air flow measurement circuits do not provide for the inexpensive generation of digital signals corresponding to the mass air flow over a sense element. In addition to requiring a separate A/D converter, these prior art mass air flow circuits must perform a squaring of the voltage applied to the bridge circuit, since it is the power dissipated by the sense element which is substantially proportional to mass air flow.